1. Field of Invention
The present invention relates generally to audio processing and more. particularly to speech enhancement using inter-microphone level differences.
2. Description of Related Art
Currently, there are many methods for reducing background noise and enhancing speech in an adverse environment. One such method is to use two or more microphones on an audio device. These microphones are in prescribed positions and allow the audio device to determine a level difference between the microphone signals. For example, due to a space difference between the microphones, the difference in times of arrival of the signals from a speech source to the microphones may be utilized to localize the speech source. Once localized, the signals can be spatially filtered to suppress the noise originating from the different directions.
In order to take advantage of the level difference between two omni-directional microphones, a speech source needs to be closer to one of the microphones. That is, in order to obtain a significant level difference, a distance from the source to a first microphone needs to be shorter than a distance from the source to a second microphone. As such, a speech source must remain in relative closeness to the microphones, especially if the microphones are in close proximity as may be required by mobile telephony applications.
A solution to the distance constraint may be obtained by using directional microphones. Using directional microphones allows a user to extend an effective level difference between the two microphones over a larger range with a narrow inter-level difference (ILD) beam. This may be desirable for applications such as push-to-talk (PTT) or videophones where a speech source is not in as close a proximity to the microphones, as for example, a telephone application.
Disadvantageously, directional microphones have numerous physical drawbacks. Typically, directional microphones are large in size and do not fit well in small telephones or cellular phones. Additionally, directional microphones are difficult to mount as they required ports in order for sounds to arrive from a plurality of directions. Slight variations in manufacturing may result in a mismatch, resulting in more expensive manufacturing and production costs.
Therefore, it is desirable to utilize the characteristics of directional microphones in a speech enhancement system, without the disadvantages of using directional microphones, themselves.